Nitinol, a class of nickel-titanium alloys, is well known for its shape memory and pseudoelastic properties. As a shape memory material, nitinol is able to undergo a reversible thermoelastic transformation between certain metallurgical phases. Generally, the thermoelastic shape memory effect allows the alloy to be shaped into a first configuration while in the relative high-temperature austenite phase, cooled below a transition temperature or temperature range at which the austenite transforms to the relative low-temperature martensite phase, deformed while in a martensitic state into a second configuration, and heated back to austenite such that the alloy transforms from the second configuration to the first configuration. The thermoelastic effect is often expressed in terms of the following "transition temperatures": M.sub.s, the temperature at which austenite begins to transform to martensite upon cooling; M.sub.f, the temperature at which the transformation from austenite to martensite is complete; A.sub.s, the temperature at which martensite begins to transform to austenite upon heating; and A.sub.f, the temperature at which the transformation from martensite to austenite is complete.
As a pseudoelastic material, nitinol is able to undergo an isothermal, reversible transformation from austenite to martensite upon the application of stress. This stress-induced transformation to martensite typically occurs at a constant temperature between A.sub.s and M.sub.d, the maximum temperature at which martensite can exist in an alloy even under stress conditions. The elasticity associated the transformation to martensite and the resulting stress-induced martensite make pseudoelastic nitinol suitable for applications requiring recoverable, isothermal deformation. For example, conventional pseudoelastic nitinol is useful for applications requiring recoverable strains of up to 8% or more. See, e.g., U.S. Pat. No. 4,935,068 to Duerig, incorporated herein by reference.
Since being discovered by William J. Buehler in 1958, the unique properties of nitinol have been applied to numerous applications. For example, as reported in C. M.
Wayman, "Some Applications of Shape-Memory Alloys," J. Metals 129 (June 1980), incorporated herein by reference, nitinol has been used for applications such as fasteners, couplings, heat engines, and various dental and medical devices. Owing to the unique mechanical properties of nitinol and its biocompatibility, the number of uses for this material in the medical field has increased dramatically in recent years.
Although conventional nitinol is known to be an elastic material, its ductility has a limit. For example, U.S. Pat. No. 4,878,954 to Dubertret et al., which is incorporated herein by reference, describes a process for improving the ductility of nitinol whereby up to 49% elongation to fracture is achieved. For some applications, however, it is desirable to employ materials having extraordinary ductilities. In addition, it is often desirable to make nitinol components in which the ductility preferentially varies with location such that ductility is highest where needed for proper application.